Plus/Minus Cyl

[Katalio]

Hi all,

Just a quick question of curiousity for the ophthalmologists! Is there a reason why OMD uses the plus cylinder form for astigmatism instead of the minus cyl? or is it just conventional for OMD's?

Thanks!

Katalio

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[rubensan]

i asked the same question and this is the response i got.

Dear Ruben
Your question is quite valid. Yes, on an "optical cross" the resultant powers are the same in either cylinder form. In plus cylider the assumption is that the lens will be manufactured with the cylinder correction on the front of the lens. In minus cylinder, the cylinder correction is placed on the back of the lens. The difference is small until you get into higher powers. Then vertex optics finds that focal points are in different locations due to placement of the cylinder correction. For over 50 years now all ophthalmic lenses are manufactured in minus cylinder. An Rx issued from a plus cylinder refraction may not, in fact, be the prescription received by the eye in the finished pair of glasses. This is why ODs use minus cylinder, to determine a "more perfect" refraction. It is my understanding that OMDs have retained the plus cylinder because of tradition, as no optical reason could ever be justified. Or perhaps OMDs acknowledge that refracting is not their function, its not important enough to be absolutely accurate.

 

[Richard_Hom]

Hi all,

Just a quick question of curiousity for the ophthalmologists! Is there a reason why OMD uses the plus cylinder form for astigmatism instead of the minus cyl? or is it just conventional for OMD's?

Thanks!

Katalio

Many years ago, surgeons did sutures onto the meridian of the plus cylinder. By knowing the plus cylinder axis, they could estimate the meridian for the suture. Although probably not a strong reason for its use, it was extremely useful.

Richard

 

[UABopt]

I was always under the impression that the reason the OMD’s tend to use plus cyl and ODs use minus cyl is mostly historic in nature and has just continued mostly out of tradition.
A minus cylinder phoropter affords better control over accommodation… it is possible for example to place the posterior focal line of an astigmat on the retina and collapse the interval of sturm with the minus cyl, with no risk that the patient will accommodate to place the circle of least confusion on the retina (if the patient were to accommodate the entire interval would move forward making the patients image even more blurry). In a plus cyl phoropter you would have to place the anterior focal line on the retina and collapse the interval with plus cyl, thus allowing the patient to accommodate to bring the entire interval forward and place the circle of least confusion on the retina (which would afford a clearer image). It is therefore much more difficult to refract using the clock dial or Madox V methods of refraction in a plus cyl phoropter.
OMD’s did not have to worry about accommodation nearly as much as OD’s as they could always use cycloplegic drops on their patients if necessary. For OD’s cycloplegia was not an option back in the days when optometry was a “drugless” profession. This is what I was told when I asked the question… and it makes sense to me.